This invention relates to a method of increasing the fatigue life and preventing stress corrosion of metal parts and, more particularly, to a method of increasing the fatigue life and preventing stress corrosion of inner, working surfaces of pressure chambers or cylinders.
Still more specifically, the present invention has particular application to the fluid end portions or fluid chambers of positive displacement pumps, such as well service or mud pumps, which are used extensively in the petroleum industry in connection with exploration and drilling for petroleum. In such applications, the corrosion-fatigue environment of the fluid chambers of the positive displacement pumps is of particular concern especially when the operating pressures of the pump causes internal stress which approach or may exceed the yield strength of the materials used in their manufacture.
Shot peening and autofrettage are two recognized methods of achieving beneficial residual compressive stresses on the surface of metal components. The autofrettage process or method is to be understood to mean for purposes of this application the process of increasing the strength of the walls forming a closed or open chamber by the application of fluid pressure in the chamber of a magnitude sufficient to deform the walls beyond the yield strength but insufficient to cause fracture and thereafter relieving such fluid pressure. The shot peening and the autofrettage methods differ in the method of application and depth of residual compressive stress. With regard to the depth of residual compressive stresses achieved on the surface of a wall, shot-peening establishes a relatively shallow residual compressive stress, on the outermost surface of the wall of a predetermined depth, whereas autofrettage achieves a residual compressive stress of considerable depth which extends into the substrate approximately an order of magnitude greater than the depth achieved by shot-peening. Furthermore, in the case of autofrettage, such method is limited to a specific component configuration and only to the inner working surfaces of thick walled pressure chambers or cylinders. This limitation is due to the nature of autofrettage which requires that areas of high stress concentrations on the inner working surfaces are initially subject to an applied internal pressure sufficiently great so that they are stressed by expansion beyond their yield point but below that required for fracture. The other working surfaces, those that were absent areas of high stress concentrations, are also stressed and deformed, but not permanently since the loading, due to such applied autofrettage pressure, is below the point of loading at which the metal ceases to deform in direct proportion to stress. Then the applied pressure is released resulting in the formation of a residual compressive stress on the areas of high stress concentrations on the inner working surfaces. While such component part is subject to such a high internal pressure there is at least a circumferential tension component and a radial compression component at any radius, the circumferential tension generally diminishing in the body of the substrate material from the inner surface of the wall of the chamber to the exterior surface of the part and is at a maximum therein at a radius separating the plastic from the elastic zone in said substrate. This tension component, unless neutralized, can produce microscopic fractures in the internal surface of the chamber wherein defects exist therein. These cracks act as nucleation sites for failure in a corrosion-fatigue environment such as may exist in the fluid chambers of positive displacement pumps.
Shot peening is a process in which compressive stresses are induced in the upper layer or exposed surface layers of metallic parts by the impingement of a stream of metal or glass shot, directed at the metal surface at high velocity and under controlled conditions, whereby increased fatigue strength and resistance to stress corrosion is imparted to the workpiece. When shot peening is performed properly, there is established a predictable amount of residual compressive stress at the peened upper layer or exposed surface layers of the metallic part and which extends for a relatively shallow depth therein; residual tensile forces are established in the region below the upper layer in the body of the substrate which upper layer is in compressive stress.
It is, therefore, an object of this invention to provide a method of increasing the fatigue life and preventing stress corrosion of inner, working surfaces of pressure chambers or cylinders.
Another object of the present invention is to provide an improved method which utilizes autofrettage but which substantially obviates the formation of fractures during the autofrettage portion of the process.
A further object of the present invention is to provide a method which utilizes autofrettage but substantially, effectively neutralizes the deleterious effect of the applied tensile component on the upper layer of the metallic part during the pressure phase of the autofrettage process.
A still further object of the present invention is to provide an improved method which includes the process of autofrettage but which lessens, if not eliminates, the propensity of material subjected thereto to crack due to the pressures required to perform said autofrettage process thereon.